Graphene is a carbon crystal in two-dimensional sheet in which carbon atoms are connected in hexagonal honeycomb shape, and multiple plate-type graphene form graphite. Therefore, when the graphite is exfoliated, graphene consisting of a single or a plurality of layers is obtained in a plate form. Graphene is a material having both the metallic properties and the non-metallic properties. As the metallic properties, there are good electrical and good thermal conductivity, and as the non-metallic properties, there are high thermal stability and high chemical inertness. Graphene can be applied to various uses such as an electric device, battery, fuel cell, the refractory material. Also, graphene or graphite is oxidized to produce graphene oxide (GO, oxidized graphene) with excellent dispersity. A non-oxidized graphene can be made without oxidation by using physical energy. Binder is added to such a graphene or graphene oxide to form ink and paste which enables patterning.
On the other hand, while graphene oxide has good dispersity, it has poor electrical conductivity so that the application to fields requiring good electrical conductivity is difficult. Accordingly, a process for reducing graphene oxide or a process for hybridizing with metal is necessary. As conventional reduction methods of graphene oxide, there are a heating method in which graphene oxide is heated at high temperature of 100 to 450° C. or 160 to 200° C. (Korean Patent No. 10-1093140B, Korean Patent Unexamined Publication No. 10-2013-0005101A), and a method in which graphene oxide is reduced with hydrazine such as phenyl hydrazine iodide or phenyl hydrazine etc. (Korean Patent Unexamined Publication No. 10-2012-0024263A, 10-2012-0084373A). Conventionally in order to hybridize graphene or graphene oxide with metal, graphene and metallic precursor etc. are reacted (Korean Patent Unexamined Publication No. 10-2013-0005101A), or hydrazine is applied after mixing graphene and metallic precursor etc. However, in case where ink in aqueous solution is formed, due to the surface tension of water, the above-mentioned reduction of graphene oxide and hybridization of graphene with metal are suitable for casting coating, not suitable for pattern-printing. In addition, when printing ink or paste prepared with solvent having a low boiling point of 120 or less are used for roll to roll (R2R) printing and screen printing, drying phenomenon and surface tension make the patterning be difficult. Also, a conventional reducing agent, such as sodium borohydride (NaBH4), borane, hypophosphite (H3PO2), citrate, hydrazine, etc. causes the high acidity of the carbon-metal composite to be harmful, and the use of aldehydes, saccharides (sugars), alcohol, polyols etc. results in low reduction rate and thus it is difficult to reduce the graphene oxide or metal oxide.